Histiocytoses

Histiocytoses are a group of disorders encompassing reactive and neoplastic conditions derived from macrophages, dendritic cells, and histiocytes. The terminology and categorization of histiocytic disorders have been a controversial issue because of the vague definition of histiocytes. Recently, however, the cell lineage of myeloid-derived macrophages and dendritic cells is becoming clear through immunophenotypic analysis. The classification based on cell lineage provides us new insight into histiocytoses, although there are still some controversies between clinical and pathological entities. This chapter reviews the current definition, etiology, clinicopathologic features, and cell lineage of histiocytic disorders involving the skin, with special attention to the representative entities: Langerhans cell histiocytosis (LCH), Rosai-Dorfman disease, juvenile xanthogranuloma, and reticulohistiocytosis